EMBRYOLOGY 



209 



Origin of Germ Cells in the Vertebrates. — In the vertebrate (back- 

 boned) animals it has not been possible to discover the reproductive cells 

 in the early divisions of the egg. In some cases, however, it seems clear 

 that the germ cells are definitely set aside long before the reproductive 

 organs appear. Thus, in a few vertebrates, the germ cells are found in 

 the early embryo, lodged among the cells of the lining of the digestive 

 tract. From this location they migrate into the outer layers of the diges- 

 tive tube, through the mesentery suspending the digestive system from 

 the back, and outward to the place where the reproductive organs subse- 

 quently appear. In Fig. 163, which is 

 purely diagrammatic, the germ cells are 

 represented as scattered along this path 

 of migration. 



Maturation. — After the germ cells 

 are set aside, they remain for a long time 

 in a relatively undifferentiated condition. 

 They are distinguishable as germ cells, 

 but they show no signs of the changes 

 which specially prepare them to take 

 part in fertilization and the formation of 

 new individuals. Often it cannot even 

 be stated whether they will become eggs 

 or spermatozoa, that" is, whether the 

 animal in which they are contained will 

 be a female or a male; yet in most 

 animals, despite their lack of recognition 

 marks, they are irrevocably destined to 

 become the one or the other. During 

 this time they divide frequently by ordi- 

 nary processes of mitosis, thereby mul- 

 tiplying in number. In this apparently 

 unspecialized'condition the reproductive 

 cells are called, in a male animal, sperma- 

 togonia (singular spermatogonium), in a 

 female, oogonia. 



Both spermatogonia and oogonia undergo thereafter a series of strik- 

 ing changes comprehended under the term maturation. The maturation 

 of germ cells in the male is called spermatogenesis, in the female oogenesis. 

 In brief, maturation consists of two rapidly succeeding cell divisions, in 

 at least one of which the behavior of the chromosomes is unlike that which 

 occurs in any other cell divisions anywhere in the body. In these divisions 

 there are many variations in detail in different species, but the fundamen- 

 tal features of the process are the same for nearly all of the higher animals. 

 The account that follows apphes to a large number of animals reproduc- 

 ing by the bi-sexual method. 



Fig. 162. — Development of the 



centrolecithal egg of the fly Miastor, 

 showing the cleavage cells (rl) at the 

 periphery and the germ cells (gc) at 

 the posterior end. (After Hegner in 

 Journal of Morphology.) 



